This invention relates to igniters.
The invention is more particularly concerned with semiconductor, surface-discharge igniters of the cavity kind, that is, in which discharge in initiated within a cavity of the igniter.
The principle behind the operation of cavity discharge igniters is that the discharge within a cavity causes an expansion of substances in the cavity, leading to projection of an ignition plasma out of the tip. Because the plasma is ejected from the igniter, ignition of a fuel/air mixture, such as in a gas-turbine enginem can occur at some distance from the igniter tip. This has some advantages, such as enabling the igniter to be located at a distance from the ignition zone, in a cooler region where the igniter will have a longer life. Previous semiconductor cavity discharge igniters have been proposed with a cylindrical pellet of semiconductor material with a central bore that defines the discharge path between two electrodes. One electrode is located within the bore recessed back from the end of the pellet, while the other electrode is provided by the outer shell of the igniter which extends laterally across the forward end of the pellet and is formed with a central orifice axially aligned with the bore through the pellet. In this way, the discharge path occurs between the central, recessed electrode and the circular edge of the orifice, over the cylindrical surface of the pellet bore. In previous igniters, the orifice is selected to have a diameter that is the same as, or smaller than, that of the semiconductive discharge cavity so that expansion of gases and vapour is confined in the cavity to a certain extent and a high velocity jet of plasma is projected through the orifice.
The usefulness of these previous igniters, however, has been limited to relatively low airflows and relatively high fuel/air ratios.